Dry paint often comes in contact with itself especially in window and door areas and, depending on its hardness, the pressure, temperature, humidity, and duration of time which the surfaces are in contact, the painted surfaces sometimes stick together. This undesirable sticking together of two painted surfaces when pressed together or placed in contact with each other is referred to as "blocking". Thus, an important characteristic of coatings is lock resistance.
The glass transition temperature (Tg) of a polymer is an inherent physical property of the monomer or monomers used to make the polymer. The Tg of a polymer determines the relative hardness or softness of the polymer. The higher the polymer's Tg the harder the polymer, and the lower the polymer's Tg the softer the polymer. The Tg of a polymer determines the physical characteristics of a film formed from a coating composition containing the polymer, as well as the minimum temperature at which the coating composition containing the polymer can be applied to a substrate to form a film.
Increasing the Tg of a polymer useful as a binder in a coating will increase the hardness of the final coating. The hardness of a coating is important because it affects other desirable properties of the coating such as block resistance.
Although an aqueous coating can be formulated with a polymer that gives the desired balance of hardness properties, it can lack adequate film formation properties; this may result in a coating which is brittle or inflexible. Adequate film formation is indicated by the formation of a continuous coating free from defects such as, for example, cracks and flakes. As used herein, "brittleness" refers to the tendency of a coating to crack or snap when subjected to a deformation, such as, for example, movement or temperature change. It is not desirable for a coating to have brittleness. It is desirable for a coating to have flexibility. As used herein, "flexibility", refers to the degree to which a coating, after drying, is able to conform to movement or deformation of its supporting surface, without cracking or flaking. The need for flexibility is demonstrated by coatings which are subjected to stress by substrates which expand or contract when exposed to changes in weather, such as, for example, bridges, building shingles, and the like.
As the Tg of the polymer used as the binder in the aqueous coating is increased, the hardness of the coating is increased, and the minimum temperatures required for the coating to form a film, herein after referred to as the "Minimum Film Formation Temperature" or "MFT" is also increased. If the polymer is selected for its ability to contribute hardness to the coating, but the coating will not form a film at the temperatures at which it is to be applied, the coating formulation is not useful. Therefore, it is necessary to add a coalescent to the coating formulation. A coalescent is an organic solvent that lowers the MFT of the polymer, thereby permitting the coating to form a useful film at a temperature below the Tg of the polymer. Coalescents, such as for example 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (Texanol), are typically employed in coating formulations at concentrations of from about 3 to about 80 percent by weight based on the weight of the polymeric binder solids.
The use of coalescents has proven to be a very useful way to solve the problem of containing certain desired film properties with high Tg polymers, which do not readily form films at desired application temperatures; however, this solution has created another problem. During the drying of a coalescent containing formulation, the organic solvents evaporate and enter into the atmosphere. In addition to the unpleasant odor associated with these organic solvents, there is growing concern about the potentially adverse environmental and health effects of many of these organic solvents.
Blends of hard and soft emulsion polymers are known in the art. EP 466,409 A1 describes a system which contains a mixture of a hard latex with Tg greater than 20.degree. C., and a soft latex with a Tg less than 15.degree. C. The blend system described in EP 466,409 A1 is disclosed to result in films with adequate film formation and hardness without the use of a coalescent.
U.S. Pat. No. 5,308,890 describes a blend of emulsion polymers containing a soft stage polymer having a Tg of less than 50.degree. C. and a hard stage polymer having a Tg of from 20.degree. C. to 160.degree. C. wherein the Tg of the soft stage polymer is lower than the Tg of the hard stage polymer and the hard stage polymer does not form a film at ambient temperature.
There is a need for an emulsion polymer for aqueous coatings which provides desirable hardness properties, adequate film formation at temperatures approaching 0.degree. C., and flexibility. In addition, it is also desirable to reduce or eliminate the amount of organic solvents in an aqueous coating without compromising physical properties of film formation.